GaN Power Transistors Gain Ground In Military And Commercial Applications

Military users help drive the development of GaN power transistors, but commercial designers are now taking advantage of the technologys many benefits.

Amplifier makers have begun to tap the benefits of gallium-nitride (GaN) RF power transistors for emerging defense and commercial applications. Thanks to this growing demand, GaN device developers are getting ready to take these wide-bandgap compound-semiconductor devices to mass production. A few may have already stepped into production mode, while others are qualifying fabrication processes and characterizing parts for mass production this year.

An example is RF Micro Devices, which is in the process of qualifying 17 new GaN devices for production. These GaN parts cover frequencies from 30 MHz to 4 GHz with output power ranging from 8 to 500 W. Five of these devices belong to the unmatched power transistor (UPT) line, while three devices are part of the matched power transistor (MPT) family. Likewise, the broadband power transistor (BPT) series will offer six devices and the power integrated-circuit (IC) category includes three high-power multichip modules (MCMs). The MCM amplifiers comprise GaN transistor die and gallium-arsenide (GaAs)-based integrated passive circuits, which are encased in a single package. Passive circuits include impedance- matching transformers, stabilization, and bias-decoupling components.

According to RFMD, a 48-V, 30-W GaN UPT transistor, dubbed the RF3931, has already achieved full production qualification. It is being shipped to multiple high-power-amplifier (HPA) manufacturers. The other four members of the UPT family are expected to go into production over the next two quarters. Ranging from 10 to 120 W, these wide-bandwidth UPT power transistors are intended for high-efficiency HPAs that target a broad range of applications, including cellular and WiMAX infrastructure, cabletelevision (CATV), military communications, public mobile radio (PMR), and radar jammers. In addition, it is eying applications that will exploit the high efficiency of its GaN transistors to reduce energy costs. In essence, RFMD has targeted five markets for its GaN power transistors: cellular base stations, industrial/emerging market, military electronic warfare (EW), military and PMR communications, and military/civilian radars.

According to Jeff Shealy, VP and General Manager for the firm's Defense and Power business unit, "Our state-of-the-art GaN process technology delivers superior RF power per square millimeter and superior RF conversion efficiency as compared to current semiconductor technologies." The supplier's 0.5-m-gate-length GaN1 process, which is optimized for power density and efficiency, has demonstrated a mean-time-to-failure (MTTF) of 30 million hours at a channel temperature of +200C and drain-source voltage (Vds) of 48 V. It boasts power density of 7.5 W/mm.

After successfully qualifying the process for its high-power GaN-on-silicon-carbide (GaN-on-SiC) high-electron-mobility-transistor (HEMT) technology, RFMD announced the formation of a GaN-foundry-service business unit (Fig. 1). According to the maker, it prefers SiC substrate for its superior thermal conductivity, low temperature-related memory effects, and high power density. By improving its GaN process, the company also plans to develop transistors that will enable it to build efficient broadband HPAs with high linearity. Its second-generation process, GaN2, is expected to be released by the end of this year while its third-generation process, GaN3, is in the R&D phase.

SiC Substrates For more than a decade, TriQuint Semiconductor has been leading GaN-on-SiC R&D efforts for the Defense Advanced Research Projects Agency (DARPA) and other military applications. The company has since extended that expertise and experience to both standard products and foundry service. Subsequently, discrete die-level, GaN-on-SiC HEMT power transistors were unveiled for both military and commercial applications. Now, the manufacturer has packaged these n-channel parts for sampling to select customers.

According to TriQuint's Marketing Manager, Mike Lincoln, the five discrete-packaged GaN-on-SiC n-channel HEMT transistors are in the process of being characterized. They will be released to production by the middle of this quarter. These GaN transistors, which cover 500 MHz to 6 GHz, are designed to deliver 5 to 60 W continuous-wave (CW) input power at 1-dB compression. Housed in a plastic package, the 5-W part offers 58 percent power-added efficiency (PAE) with 11 dB gain at 6 GHz. The higher-power versions are encased in ceramic packages.

For defense-communications applications like joint tactical radio service (JTRS) and counter-improvised-explosive- device (IED) EW systems, TriQuint has released a GaN-based power amplifier (PA) in a flange package. This PA offers at least 8 W of output power at 1-dB compression with 40 percent PAE from 30 to 3000 MHz. Because the TGA2540-FL is classified as XI(c) under International Traffic in Arms Regulations (ITAR), it is available to US-based manufacturers. An export license is required for international shipments.

Another SiC backerCree, Inc.is in production mode with its general-purpose GaN-on-SiC HEMT transistors along with those tailored for C-band and third-generation/Long Term Evolution (3G/LTE) and WiMAX applications. The seven unmatched HEMT transistors in the general-purpose series offer saturated output power from 10 to 220 W. The three lower-power models handle signals to 6 GHz while the four higher-power members are crafted for operation to 4 GHz. Two recent additionsthe CGH40120F and CGH40180PPclaim to provide high efficiency and power under continuous-wave (CW) operation. For both new general-purpose parts, PAE is rated at 70 percent in Class AB amplifier configuration.

Introduced about a year ago, the two C-band HEMT devices also are in production, states Cree's Director of Marketing, Tom Dekker. "Military has been the early adopter of the technology and more applications are taking advantage of GaN transistors. We are growing handsomely and the business is expanding," Dekker asserts. In the commercial world, telecommunications and medical applications are looking for amplifiers that offer a smaller footprint, high efficiency, and high bandwidth. According to Dekker, GaN technology meets those demands. Cree is planning to add three to five new parts every quarter this year.

Presently, there are only two GaN-based monolithic-microwave-integrated- circuit (MMIC) amplifiers in the product portfolio. Yet the manufacturer is planning to expand the line this year with MMICs tailored for higher power and efficiency. Currently, the CMPA0060005 is a wideband 5-W distributed amplifier operating from DC to 6 GHz. Its sibling, the CMPA2560025, is a 25-W reactively matched amplifier operating from 2.5 to 6.0 GHz. The MMICs can be combined to achieve higher power and gain. From 2.5 to 6.0 GHz, for example, a pair of CMP2560025s driven by the CMPA0060005 can offer more than 40 dB gain with output power to 50 W.

Eudyna Devices, Inc. is another proponent of SiC substrate for GaN HEMTs. An early entrant into this space, the Japanese supplier has targeted cellular-base-station and WiMAX-infrastructure applications for its line of GaN HEMTs. Eudyna's GaN HEMTs, which can deliver more than 100 W from a single package, are commercially available. Among the latest additions are the EGNB010MK, EGNB030MK, EGNB045MK, and EGNB070MK with nominal output powers of 12.6, 45, 56, and 90 W, respectively. They operate to 3.5 GHz.

In volume production of GaN devices since 2008, Toshiba America Electronic Components, Inc. continues to expand its GaN HEMT product portfolio. At last year's International Microwave Symposium (IMS), the company unveiled its first commercial C-band GaN HEMT for satellite applications. The TGI 7785-120L operates from 7.7 to 8.5 GHz with output power of 120 W. This internally matched transistor is designed to achieve high power density.

TAEC also has released two additional parts. The Ku-band TGI1314-50L operates from 13.75 to 14.5 GHz with 50 W output power while the X-band TGI1011-50-771 covers 11.3 to 11.5 GHz with 50 W output power. The Ku-band part is aimed at satellite applications while the X-band unit targets industrial markets. According to TAEC, devices in other bands with higher output power are in the works.

GaN-On-Silicon HEMTs A unique approach is taken by Nitronex, which has adopted 100-mm silicon as the substrate of choice for its GaN devices. High-quality silicon substrates are affordable, scalable, consistent, and plentiful, thanks to more than 50 years of use in the microelectronics industry. In addition, the yield is high. Nitronex asserts that it can potentially scale its GaN-on-Si production to 150-mm wafers.

The firm also claims that GaN-on-Si HEMT devices are inherently more cost effective than SiC substrates. Over the years, the company has developed nearly a dozen different GaN-on-Si HEMT transistors for both military and commercial applicationswith all still in production. In fact, its first-generation GaN-on-Si platform was formally qualified and released to production in October 2006. To boost the power level to 200 W for applications to 1.2 GHz, the manufacturer recently combined two 100-W devices in a single package. Offering simultaneously high gain and efficiency from 14 to 28 V, the 200-W NPT1007 comes in a thermally enhanced, ceramic-air-cavity bolt-down package (Fig. 2).

The supplier also is readying highly integrated MMIC amplifiers and high-voltage GaN transistors for radar applications. Initially, it is developing 48-V, 50-O input/output-matched MMIC amplifiers in the 10-to-20-W range with plans for more than 300 W in the future. With the rapid adoption of GaN-based RF power transistors, such suppliers also have been urged by amplifier designers to develop device models for their respective parts. An example is Nitronex and Modelithics, which inked a partnership to develop nonlinear models for Nitronex's high-power GaN-on-Si HEMT devices.